A primary objective of microelectronic circuit design is to develop compact components that consume minimal power. As integrated circuit chips shrink in size, it is imperative that the power supplies that provide power for these chips also become compact, yet achieve high efficiencies. With the advent of hand held computers, the need for compact and efficient power supply topologies is acute because these computers are very compact and run on batteries whose useful lifetimes are relatively short.
To reduce power consumption, integrated circuit manufacturers have developed components that require 3.3 volts rather than the typical 5 volt input supply voltage. However, because not all components are available with 3.3 volt operation, some systems require both a 3.3 and 5 volt power supply. The power supply must, therefore, provide two regulated outputs.
Certain applications, such as portable computer systems, require a power supply that can provide two regulated output voltages from an input voltage source that varies from voltages above the higher regulated output to voltages between the regulated outputs. Typically, portable computer systems have battery packs that provide D.C. voltages ranging from 4 to 15 volts. To provide a 3.3 and 5 volt regulated output, the power supply must be capable of converting the input voltage to both a higher and a lower voltage.
Existing power supplies that meet the desired performance characteristics typically use transformers to provide regulated outputs. Such power supplies, however, suffer from several drawbacks. For example, in power supplies that use transformers, it is very difficult to turn off one of the regulated outputs to save power consumption (as is sometimes desired in 3.3/5 volt systems). Further, transformers impair efficiency because energy is lost in the transfer of energy between the primary and secondary windings. Finally, transformers require more space and therefore are less desirable in compact circuit layouts.
Power supplies without transformers do exist, yet these power supplies are relatively inefficient and generally do not provide two regulated outputs. For example Linear Technology Corporation has published an application note regarding a switching regulator that provides a constant 5 volt output from a 3.5 to 40 volt input without using transformers. This particular regulator requires two inductors and employs capacitive coupling to produce a single 5 volt output. Because of the losses in the circuit elements and the use of capacitive coupling, the efficiency of this regulator is below 80%. Producing only one regulated output, this regulator is unsuitable for applications requiring two regulated outputs.
It is desirable, therefore, to have a power supply without transformers that is capable of efficiently providing two regulating outputs from a varying input voltage.
In accordance with the present invention, a high efficiency power supply produces two regulated voltage outputs (such as 3.3 and 5.0 volts) from an unregulated input voltage. The preferred circuit features an efficiency greater than 80% and is well adapted for miniaturized implementation.
In greater detail, the preferred embodiment includes a 4-15 volt battery pack, a first (5.0 volt) output, a second (3.3 volt) output, and a ground. The positive terminal of the battery is connected directly to the second output. The 3.3 volt differential with ground is maintained by a voltage regulator that couples the negative terminal of the battery pack to ground (i.e. ground floats relative to the battery). Another voltage regulator couples the positive terminal of the battery pack to the first output, and maintains this first output at 5.0 volts relative to ground. This latter regulator is capable of boost operation, permitting a 5.0 volt output to be maintained even if the battery pack voltage is less than this value.
The foregoing and other advantages and features will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.